Nowadays, global positioning system (GPS) receivers are widely used in many applications to provide a position for a moving object according to satellite signals. FIG. 1 shows a block diagram of a conventional GPS receiver 100. As shown in FIG. 1, after a mixed signal including multiple satellite signals is received via an antenna, the mixed signal is processed through a pre-amplifier 102 for amplifying the mixed signal, a radio frequency to Intermediate frequency (RF/IF) convertor 104 for converting a frequency of the mixed signal from RF to IF, and an analog to digital (ND) convertor 106 for converting the mixed signal from analog to digital. Finally, the mixed signal is converted into a digital baseband signal. The digital baseband signal is then provided to a digital baseband processor 108.
The digital baseband processor 108 captures the multiple satellite signals by demodulating the digital baseband signal, tracks the multiple satellite signals, and gets the navigation information of the multiple satellite signals. Generally, the navigation information includes, but not limit to, a receiving time of a Pseudorange, a GPS carrier phase, and a Doppler frequency of the satellite signal. The digital baseband processor 108 includes a digital delay lock loop (DDLL), a digital phase lock loop (DPLL) and a digital frequency lock loop (DFLL) to measure the receiving time of the Pseudorange, the GPS carrier phase and the Doppler frequency of the satellite signals respectively. The digital baseband processor 108 outputs the navigation information into a navigation processor 110 for calculating the current position, velocity and time of a moving object (PVT information). Finally, the PVT information is displayed on a display unit 120.
In the digital baseband processor 108, the Doppler frequency of the satellite signal measured by the DFLL can be used for correcting the measuring value of the DDLL. However, if the received satellite signal is relatively weak (abnormal condition), the Doppler frequency measured by the DFLL may deviate from the actual Doppler frequency, which affects correcting the measuring value of the DDLL. As a result, the accuracy and stability of the GPS receiver 100 may be decreased.
FIG. 2 shows a diagram of a Doppler frequency of a satellite signal measured by the DFLL varying with respect to time under normal condition and abnormal condition. Line 202 represents the Doppler frequency measured by the DFLL varying with respect to time under normal condition and line 204 represents the Doppler frequency measured by the DFLL varying with respect to time under abnormal condition. As shown in FIG. 2, the measured value of the Doppler frequency declines smoothly with respect to the time under normal condition. However, the measured value of the Doppler frequency does not decline smoothly around a point of time 2450 s under abnormal condition. Therefore, it is to a system that detects whether the measured value of the Doppler frequency is deviate from the actual value and minimizes the deviation of measured Doppler frequency under abnormal condition according to the corresponding detection that the present invention is directed to.